This document describes the CommandLine argument processing library. It will
show you how to use it, and what it can do. The CommandLine library uses a
declarative approach to specifying the command line options that your program
takes. By default, these options declarations implicitly hold the value parsed
for the option declared (of course this can be changed).

Although there are a lot of command line argument parsing libraries out
there in many different languages, none of them fit well with what I needed. By
looking at the features and problems of other libraries, I designed the
CommandLine library to have the following features:

Speed: The CommandLine library is very quick and uses little resources. The
parsing time of the library is directly proportional to the number of
arguments parsed, not the number of options recognized. Additionally,
command line argument values are captured transparently into user defined
global variables, which can be accessed like any other variable (and with the
same performance).

Type Safe: As a user of CommandLine, you don’t have to worry about
remembering the type of arguments that you want (is it an int? a string? a
bool? an enum?) and keep casting it around. Not only does this help prevent
error prone constructs, it also leads to dramatically cleaner source code.

No subclasses required: To use CommandLine, you instantiate variables that
correspond to the arguments that you would like to capture, you don’t
subclass a parser. This means that you don’t have to write any
boilerplate code.

Globally accessible: Libraries can specify command line arguments that are
automatically enabled in any tool that links to the library. This is
possible because the application doesn’t have to keep a list of arguments to
pass to the parser. This also makes supporting dynamically loaded options
trivial.

Cleaner: CommandLine supports enum and other types directly, meaning that
there is less error and more security built into the library. You don’t have
to worry about whether your integral command line argument accidentally got
assigned a value that is not valid for your enum type.

Extensible: It is very simple to add a new argument type to CommandLine.
Simply specify the parser that you want to use with the command line option
when you declare it. Custom parsers are no problem.

Labor Saving: The CommandLine library cuts down on the amount of grunt work
that you, the user, have to do. For example, it automatically provides a
-help option that shows the available command line options for your tool.
Additionally, it does most of the basic correctness checking for you.

This section of the manual runs through a simple CommandLine’ification of a
basic compiler tool. This is intended to show you how to jump into using the
CommandLine library in your own program, and show you some of the cool things it
can do.

To start out, you need to include the CommandLine header file into your program:

#include"llvm/Support/CommandLine.h"

Additionally, you need to add this as the first line of your main program:

… which actually parses the arguments and fills in the variable declarations.

Now that you are ready to support command line arguments, we need to tell the
system which ones we want, and what type of arguments they are. The CommandLine
library uses a declarative syntax to model command line arguments with the
global variable declarations that capture the parsed values. This means that
for every command line option that you would like to support, there should be a
global variable declaration to capture the result. For example, in a compiler,
we would like to support the Unix-standard ‘-o<filename>’ option to specify
where to put the output. With the CommandLine library, this is represented like
this:

This declares a global variable “OutputFilename” that is used to capture the
result of the “o” argument (first parameter). We specify that this is a
simple scalar option by using the “cl::opt” template (as opposed to the
“cl::list” template), and tell the CommandLine library that the data
type that we are parsing is a string.

The second and third parameters (which are optional) are used to specify what to
output for the “-help” option. In this case, we get a line that looks like
this:

Because we specified that the command line option should parse using the
string data type, the variable declared is automatically usable as a real
string in all contexts that a normal C++ string object may be used. For
example:

There are many different options that you can use to customize the command line
option handling library, but the above example shows the general interface to
these options. The options can be specified in any order, and are specified
with helper functions like cl::desc(…), so there are no positional
dependencies to remember. The available options are discussed in detail in the
Reference Guide.

Continuing the example, we would like to have our compiler take an input
filename as well as an output filename, but we do not want the input filename to
be specified with a hyphen (ie, not -filename.c). To support this style of
argument, the CommandLine library allows for positional arguments to be
specified for the program. These positional arguments are filled with command
line parameters that are not in option form. We use this feature like this:

This declaration indicates that the first positional argument should be treated
as the input filename. Here we use the cl::init option to specify an initial
value for the command line option, which is used if the option is not specified
(if you do not specify a cl::init modifier for an option, then the default
constructor for the data type is used to initialize the value). Command line
options default to being optional, so if we would like to require that the user
always specify an input filename, we would add the cl::Required flag, and we
could eliminate the cl::init modifier, like this:

By simply adding the cl::Required flag, the CommandLine library will
automatically issue an error if the argument is not specified, which shifts all
of the command line option verification code out of your application into the
library. This is just one example of how using flags can alter the default
behaviour of the library, on a per-option basis. By adding one of the
declarations above, the -help option synopsis is now extended to:

In addition to input and output filenames, we would like the compiler example to
support three boolean flags: “-f” to force writing binary output to a
terminal, “--quiet” to enable quiet mode, and “-q” for backwards
compatibility with some of our users. We can support these by declaring options
of boolean type like this:

This does what you would expect: it declares three boolean variables
(“Force”, “Quiet”, and “Quiet2”) to recognize these options. Note
that the “-q” option is specified with the “cl::Hidden” flag. This
modifier prevents it from being shown by the standard “-help” output (note
that it is still shown in the “-help-hidden” output).

The CommandLine library uses a different parser for different data types.
For example, in the string case, the argument passed to the option is copied
literally into the content of the string variable… we obviously cannot do that
in the boolean case, however, so we must use a smarter parser. In the case of
the boolean parser, it allows no options (in which case it assigns the value of
true to the variable), or it allows the values “true” or “false” to be
specified, allowing any of the following inputs:

… you get the idea. The bool parser just turns the string values into
boolean values, and rejects things like ‘compiler-f=foo’. Similarly, the
float, double, and int parsers work like you would expect, using the
‘strtol’ and ‘strtod’ C library calls to parse the string value into the
specified data type.

This brief example has shown you how to use the ‘cl::opt’ class to parse
simple scalar command line arguments. In addition to simple scalar arguments,
the CommandLine library also provides primitives to support CommandLine option
aliases, and lists of options.

So far, the example works well, except for the fact that we need to check the
quiet condition like this now:

...if(!Quiet&&!Quiet2)printInformationalMessage(...);...

… which is a real pain! Instead of defining two values for the same
condition, we can use the “cl::alias” class to make the “-q” option an
alias for the “-quiet” option, instead of providing a value itself:

The third line (which is the only one we modified from above) defines a “-q”
alias that updates the “Quiet” variable (as specified by the cl::aliasopt
modifier) whenever it is specified. Because aliases do not hold state, the only
thing the program has to query is the Quiet variable now. Another nice
feature of aliases is that they automatically hide themselves from the -help
output (although, again, they are still visible in the -help-hiddenoutput).

Now the application code can simply use:

...if(!Quiet)printInformationalMessage(...);...

… which is much nicer! The “cl::alias” can be used to specify an
alternative name for any variable type, and has many uses.

So far we have seen how the CommandLine library handles builtin types like
std::string, bool and int, but how does it handle things it doesn’t
know about, like enums or ‘int*’s?

The answer is that it uses a table-driven generic parser (unless you specify
your own parser, as described in the Extension Guide). This parser maps
literal strings to whatever type is required, and requires you to tell it what
this mapping should be.

Let’s say that we would like to add four optimization levels to our optimizer,
using the standard flags “-g”, “-O0”, “-O1”, and “-O2”. We
could easily implement this with boolean options like above, but there are
several problems with this strategy:

A user could specify more than one of the options at a time, for example,
“compiler-O3-O2”. The CommandLine library would not be able to catch
this erroneous input for us.

We would have to test 4 different variables to see which ones are set.

This doesn’t map to the numeric levels that we want… so we cannot easily
see if some level >= “-O1” is enabled.

To cope with these problems, we can use an enum value, and have the CommandLine
library fill it in with the appropriate level directly, which is used like this:

This declaration defines a variable “OptimizationLevel” of the
“OptLevel” enum type. This variable can be assigned any of the values that
are listed in the declaration. The CommandLine library enforces that
the user can only specify one of the options, and it ensure that only valid enum
values can be specified. The “clEnumVal” macros ensure that the command
line arguments matched the enum values. With this option added, our help output
now is:

In this case, it is sort of awkward that flag names correspond directly to enum
names, because we probably don’t want a enum definition named “g” in our
program. Because of this, we can alternatively write this example like this:

By using the “clEnumValN” macro instead of “clEnumVal”, we can directly
specify the name that the flag should get. In general a direct mapping is nice,
but sometimes you can’t or don’t want to preserve the mapping, which is when you
would use it.

Another useful argument form is a named alternative style. We shall use this
style in our compiler to specify different debug levels that can be used.
Instead of each debug level being its own switch, we want to support the
following options, of which only one can be specified at a time:
“--debug-level=none”, “--debug-level=quick”,
“--debug-level=detailed”. To do this, we use the exact same format as our
optimization level flags, but we also specify an option name. For this case,
the code looks like this:

This definition defines an enumerated command line variable of type “enumDebugLev”, which works exactly the same way as before. The difference here is
just the interface exposed to the user of your program and the help output by
the “-help” option:

Again, the only structural difference between the debug level declaration and
the optimization level declaration is that the debug level declaration includes
an option name ("debug_level"), which automatically changes how the library
processes the argument. The CommandLine library supports both forms so that you
can choose the form most appropriate for your application.

Now that we have the standard run-of-the-mill argument types out of the way,
lets get a little wild and crazy. Lets say that we want our optimizer to accept
a list of optimizations to perform, allowing duplicates. For example, we
might want to run: “compiler-dce-constprop-inline-dce-strip”. In this
case, the order of the arguments and the number of appearances is very
important. This is what the “cl::list” template is for. First, start by
defining an enum of the optimizations that you would like to perform:

enumOpts{// 'inline' is a C++ keyword, so name it 'inlining'dce,constprop,inlining,strip};

Note that the “cl::list” template is completely general and may be used with
any data types or other arguments that you can use with the “cl::opt”
template. One especially useful way to use a list is to capture all of the
positional arguments together if there may be more than one specified. In the
case of a linker, for example, the linker takes several ‘.o’ files, and
needs to capture them into a list. This is naturally specified as:

This variable works just like a “vector<string>” object. As such, accessing
the list is simple, just like above. In this example, we used the
cl::OneOrMore modifier to inform the CommandLine library that it is an error
if the user does not specify any .o files on our command line. Again, this
just reduces the amount of checking we have to do.

Instead of collecting sets of options in a list, it is also possible to gather
information for enum values in a bit vector. The representation used by the
cl::bits class is an unsigned integer. An enum value is represented by a
0/1 in the enum’s ordinal value bit position. 1 indicating that the enum was
specified, 0 otherwise. As each specified value is parsed, the resulting enum’s
bit is set in the option’s bit vector:

bits|=1<<(unsigned)enum;

Options that are specified multiple times are redundant. Any instances after
the first are discarded.

As our program grows and becomes more mature, we may decide to put summary
information about what it does into the help output. The help output is styled
to look similar to a Unix man page, providing concise information about a
program. Unix man pages, however often have a description about what the
program does. To add this to your CommandLine program, simply pass a third
argument to the cl::ParseCommandLineOptions call in main. This additional
argument is then printed as the overview information for your program, allowing
you to include any additional information that you want. For example:

If our program has a large number of options it may become difficult for users
of our tool to navigate the output of -help. To alleviate this problem we
can put our options into categories. This can be done by declaring option
categories (cl::OptionCategory objects) and then placing our options into
these categories using the cl::cat option attribute. For example:

Now that you know the basics of how to use the CommandLine library, this section
will give you the detailed information you need to tune how command line options
work, as well as information on more “advanced” command line option processing
capabilities.

Positional arguments are those arguments that are not named, and are not
specified with a hyphen. Positional arguments should be used when an option is
specified by its position alone. For example, the standard Unix grep tool
takes a regular expression argument, and an optional filename to search through
(which defaults to standard input if a filename is not specified). Using the
CommandLine library, this would be specified as:

… and the resultant program could be used just like the standard grep
tool.

Positional arguments are sorted by their order of construction. This means that
command line options will be ordered according to how they are listed in a .cpp
file, but will not have an ordering defined if the positional arguments are
defined in multiple .cpp files. The fix for this problem is simply to define
all of your positional arguments in one .cpp file.

Sometimes you may want to specify a value to your positional argument that
starts with a hyphen (for example, searching for ‘-foo’ in a file). At
first, you will have trouble doing this, because it will try to find an argument
named ‘-foo’, and will fail (and single quotes will not save you). Note
that the system grep has the same problem:

The solution for this problem is the same for both your tool and the system
version: use the ‘--‘ marker. When the user specifies ‘--‘ on the
command line, it is telling the program that all options after the ‘--‘
should be treated as positional arguments, not options. Thus, we can use it
like this:

Sometimes an option can affect or modify the meaning of another option. For
example, consider gcc’s -xLANG option. This tells gcc to ignore the
suffix of subsequent positional arguments and force the file to be interpreted
as if it contained source code in language LANG. In order to handle this
properly, you need to know the absolute position of each argument, especially
those in lists, so their interaction(s) can be applied correctly. This is also
useful for options like -llibname which is actually a positional argument
that starts with a dash.

So, generally, the problem is that you have two cl::list variables that
interact in some way. To ensure the correct interaction, you can use the
cl::list::getPosition(optnum) method. This method returns the absolute
position (as found on the command line) of the optnum item in the
cl::list.

Note that, for compatibility reasons, the cl::opt also supports an
unsignedgetPosition() option that will provide the absolute position of
that option. You can apply the same approach as above with a cl::opt and a
cl::list option as you can with two lists.

The cl::ConsumeAfterformatting option is used to construct programs that
use “interpreter style” option processing. With this style of option
processing, all arguments specified after the last positional argument are
treated as special interpreter arguments that are not interpreted by the command
line argument.

As a concrete example, lets say we are developing a replacement for the standard
Unix Bourne shell (/bin/sh). To run /bin/sh, first you specify options
to the shell itself (like -x which turns on trace output), then you specify
the name of the script to run, then you specify arguments to the script. These
arguments to the script are parsed by the Bourne shell command line option
processor, but are not interpreted as options to the shell itself. Using the
CommandLine library, we would specify this as:

At runtime, if we run our new shell replacement as `spiffysh-xtest.sh-a-x-ybar’, the Trace variable will be set to true, the Script variable
will be set to “test.sh”, and the Argv list will contain ["-a","-x","-y","bar"], because they were specified after the last positional argument
(which is the script name).

There are several limitations to when cl::ConsumeAfter options can be
specified. For example, only one cl::ConsumeAfter can be specified per
program, there must be at least one positional argument specified, there must
not be any cl::list positional arguments, and the cl::ConsumeAfter option
should be a cl::list option.

By default, all command line options automatically hold the value that they
parse from the command line. This is very convenient in the common case,
especially when combined with the ability to define command line options in the
files that use them. This is called the internal storage model.

Sometimes, however, it is nice to separate the command line option processing
code from the storage of the value parsed. For example, lets say that we have a
‘-debug’ option that we would like to use to enable debug information across
the entire body of our program. In this case, the boolean value controlling the
debug code should be globally accessible (in a header file, for example) yet the
command line option processing code should not be exposed to all of these
clients (requiring lots of .cpp files to #includeCommandLine.h).

To do this, set up your .h file with your option, like this for example:

// DebugFlag.h - Get access to the '-debug' command line option//// DebugFlag - This boolean is set to true if the '-debug' command line option// is specified. This should probably not be referenced directly, instead, use// the DEBUG macro below.//externboolDebugFlag;// DEBUG macro - This macro should be used by code to emit debug information.// In the '-debug' option is specified on the command line, and if this is a// debug build, then the code specified as the option to the macro will be// executed. Otherwise it will not be.#ifdef NDEBUG#define LLVM_DEBUG(X)#else#define LLVM_DEBUG(X) do { if (DebugFlag) { X; } } while (0)#endif

This allows clients to blissfully use the LLVM_DEBUG() macro, or the
DebugFlag explicitly if they want to. Now we just need to be able to set
the DebugFlag boolean when the option is set. To do this, we pass an
additional argument to our command line argument processor, and we specify where
to fill in with the cl::location attribute:

In the above example, we specify “true” as the second argument to the
cl::opt template, indicating that the template should not maintain a copy of
the value itself. In addition to this, we specify the cl::location
attribute, so that DebugFlag is automatically set.

This section describes the basic attributes that you can specify on options.

The option name attribute (which is required for all options, except
positional options) specifies what the option name is. This option is
specified in simple double quotes:

cl::opt<bool>Quiet("quiet");

The cl::desc attribute specifies a description for the option to be
shown in the -help output for the program. This attribute supports
multi-line descriptions with lines separated by ‘n’.

The cl::value_desc attribute specifies a string that can be used to
fine tune the -help output for a command line option. Look here for an
example.

The cl::init attribute specifies an initial value for a scalar
option. If this attribute is not specified then the command line option value
defaults to the value created by the default constructor for the
type.

Warning

If you specify both cl::init and cl::location for an option, you
must specify cl::location first, so that when the command-line parser
sees cl::init, it knows where to put the initial value. (You will get an
error at runtime if you don’t put them in the right order.)

The cl::location attribute where to store the value for a parsed command
line option if using external storage. See the section on Internal vs
External Storage for more information.

The cl::aliasopt attribute specifies which option a cl::alias option is
an alias for.

The cl::values attribute specifies the string-to-value mapping to be used
by the generic parser. It takes a list of (option, value, description)
triplets that specify the option name, the value mapped to, and the
description shown in the -help for the tool. Because the generic parser
is used most frequently with enum values, two macros are often useful:

The clEnumVal macro is used as a nice simple way to specify a triplet
for an enum. This macro automatically makes the option name be the same as
the enum name. The first option to the macro is the enum, the second is
the description for the command line option.

The clEnumValN macro is used to specify macro options where the option
name doesn’t equal the enum name. For this macro, the first argument is
the enum value, the second is the flag name, and the second is the
description.

You will get a compile time error if you try to use cl::values with a parser
that does not support it.

The cl::multi_val attribute specifies that this option takes has multiple
values (example: -sectalignsegnamesectnamesectvalue). This attribute
takes one unsigned argument - the number of values for the option. This
attribute is valid only on cl::list options (and will fail with compile
error if you try to use it with other option types). It is allowed to use all
of the usual modifiers on multi-valued options (besides
cl::ValueDisallowed, obviously).

The cl::cat attribute specifies the option category that the option
belongs to. The category should be a cl::OptionCategory object.

Option modifiers are the flags and expressions that you pass into the
constructors for cl::opt and cl::list. These modifiers give you the
ability to tweak how options are parsed and how -help output is generated to
fit your application well.

These options fall into five main categories:

Hiding an option from -help output

Controlling the number of occurrences required and allowed

Controlling whether or not a value must be specified

Controlling other formatting options

Miscellaneous option modifiers

It is not possible to specify two options from the same category (you’ll get a
runtime error) to a single option, except for options in the miscellaneous
category. The CommandLine library specifies defaults for all of these settings
that are the most useful in practice and the most common, which mean that you
usually shouldn’t have to worry about these.

This group of options is used to control how many time an option is allowed (or
required) to be specified on the command line of your program. Specifying a
value for this setting allows the CommandLine library to do error checking for
you.

The allowed values for this option group are:

The cl::Optional modifier (which is the default for the cl::opt and
cl::alias classes) indicates that your program will allow either zero or
one occurrence of the option to be specified.

The cl::ZeroOrMore modifier (which is the default for the cl::list
class) indicates that your program will allow the option to be specified zero
or more times.

The cl::Required modifier indicates that the specified option must be
specified exactly one time.

The cl::OneOrMore modifier indicates that the option must be specified at
least one time.

If an option is not specified, then the value of the option is equal to the
value specified by the cl::init attribute. If the cl::init attribute is
not specified, the option value is initialized with the default constructor for
the data type.

If an option is specified multiple times for an option of the cl::opt class,
only the last value will be retained.

This group of options is used to control whether or not the option allows a
value to be present. In the case of the CommandLine library, a value is either
specified with an equal sign (e.g. ‘-index-depth=17’) or as a trailing
string (e.g. ‘-oa.out’).

The allowed values for this option group are:

The cl::ValueOptional modifier (which is the default for bool typed
options) specifies that it is acceptable to have a value, or not. A boolean
argument can be enabled just by appearing on the command line, or it can have
an explicit ‘-foo=true’. If an option is specified with this mode, it is
illegal for the value to be provided without the equal sign. Therefore
‘-footrue’ is illegal. To get this behavior, you must use
the cl::ValueRequired modifier.

The cl::ValueRequired modifier (which is the default for all other types
except for unnamed alternatives using the generic parser) specifies that a
value must be provided. This mode informs the command line library that if an
option is not provides with an equal sign, that the next argument provided
must be the value. This allows things like ‘-oa.out’ to work.

The cl::ValueDisallowed modifier (which is the default for unnamed
alternatives using the generic parser) indicates that it is a runtime error
for the user to specify a value. This can be provided to disallow users from
providing options to boolean options (like ‘-foo=true’).

In general, the default values for this option group work just like you would
want them to. As mentioned above, you can specify the cl::ValueDisallowed
modifier to a boolean argument to restrict your command line parser. These
options are mostly useful when extending the library.

The formatting option group is used to specify that the command line option has
special abilities and is otherwise different from other command line arguments.
As usual, you can only specify one of these arguments at most.

The cl::NormalFormatting modifier (which is the default all options)
specifies that this option is “normal”.

The cl::Positional modifier specifies that this is a positional argument
that does not have a command line option associated with it. See the
Positional Arguments section for more information.

The cl::Prefix modifier specifies that this option prefixes its value.
With ‘Prefix’ options, the equal sign does not separate the value from the
option name specified. Instead, the value is everything after the prefix,
including any equal sign if present. This is useful for processing odd
arguments like -lmalloc and -L/usr/lib in a linker tool or
-DNAME=value in a compiler tool. Here, the ‘l’, ‘D’ and ‘L’
options are normal string (or list) options, that have the cl::Prefix
modifier added to allow the CommandLine library to recognize them. Note that
cl::Prefix options must not have the cl::ValueDisallowed modifier
specified.

The cl::Grouping modifier is used to implement Unix-style tools (like
ls) that have lots of single letter arguments, but only require a single
dash. For example, the ‘ls-labF’ command actually enables four different
options, all of which are single letters. Note that cl::Grouping options
cannot have values.

The CommandLine library does not restrict how you use the cl::Prefix or
cl::Grouping modifiers, but it is possible to specify ambiguous argument
settings. Thus, it is possible to have multiple letter options that are prefix
or grouping options, and they will still work as designed.

To do this, the CommandLine library uses a greedy algorithm to parse the input
option into (potentially multiple) prefix and grouping options. The strategy
basically looks like this:

The miscellaneous option modifiers are the only flags where you can specify more
than one flag from the set: they are not mutually exclusive. These flags
specify boolean properties that modify the option.

The cl::CommaSeparated modifier indicates that any commas specified for an
option’s value should be used to split the value up into multiple values for
the option. For example, these two options are equivalent when
cl::CommaSeparated is specified: “-foo=a-foo=b-foo=c” and
“-foo=a,b,c”. This option only makes sense to be used in a case where the
option is allowed to accept one or more values (i.e. it is a cl::list
option).

The cl::PositionalEatsArgs modifier (which only applies to positional
arguments, and only makes sense for lists) indicates that positional argument
should consume any strings after it (including strings that start with a “-“)
up until another recognized positional argument. For example, if you have two
“eating” positional arguments, “pos1” and “pos2”, the string “-pos1-foo-barbaz-pos2-bork” would cause the “-foo-bar-baz” strings to
be applied to the “-pos1” option and the “-bork” string to be applied
to the “-pos2” option.

The cl::Sink modifier is used to handle unknown options. If there is at
least one option with cl::Sink modifier specified, the parser passes
unrecognized option strings to it as values instead of signaling an error. As
with cl::CommaSeparated, this modifier only makes sense with a cl::list
option.

Some systems, such as certain variants of Microsoft Windows and some older
Unices have a relatively low limit on command-line length. It is therefore
customary to use the so-called ‘response files’ to circumvent this
restriction. These files are mentioned on the command-line (using the “@file”)
syntax. The program reads these files and inserts the contents into argv,
thereby working around the command-line length limits.

The cl::getRegisteredOptions function is designed to give a programmer
access to declared non-positional command line options so that how they appear
in -help can be modified prior to calling cl::ParseCommandLineOptions.
Note this method should not be called during any static initialisation because
it cannot be guaranteed that all options will have been initialised. Hence it
should be called from main.

This function can be used to gain access to options declared in libraries that
the tool writter may not have direct access to.

The function retrieves a StringMap that maps the option
string (e.g. -help) to an Option*.

Here is an example of how the function could be used:

usingnamespacellvm;intmain(intargc,char**argv){cl::OptionCategoryAnotherCategory("Some options");StringMap<cl::Option*>&Map=cl::getRegisteredOptions();//Unhide useful option and put it in a different categoryassert(Map.count("print-all-options")>0);Map["print-all-options"]->setHiddenFlag(cl::NotHidden);Map["print-all-options"]->setCategory(AnotherCategory);//Hide an option we don't want to seeassert(Map.count("enable-no-infs-fp-math")>0);Map["enable-no-infs-fp-math"]->setHiddenFlag(cl::Hidden);//Change --version to --show-versionassert(Map.count("version")>0);Map["version"]->setArgStr("show-version");//Change --help descriptionassert(Map.count("help")>0);Map["help"]->setDescription("Shows help");cl::ParseCommandLineOptions(argc,argv,"This is a small program to demo the LLVM CommandLine API");...}

The cl::ParseCommandLineOptions function is designed to be called directly
from main, and is used to fill in the values of all of the command line
option variables once argc and argv are available.

The cl::ParseCommandLineOptions function requires two parameters (argc
and argv), but may also take an optional third parameter which holds
additional extra text to emit when the -help option is invoked.

The cl::ParseEnvironmentOptions function has mostly the same effects as
cl::ParseCommandLineOptions, except that it is designed to take values for
options from an environment variable, for those cases in which reading the
command line is not convenient or desired. It fills in the values of all the
command line option variables just like cl::ParseCommandLineOptions does.

It takes four parameters: the name of the program (since argv may not be
available, it can’t just look in argv[0]), the name of the environment
variable to examine, and the optional additional extra text to emit when the
-help option is invoked.

cl::ParseEnvironmentOptions will break the environment variable’s value up
into words and then process them using cl::ParseCommandLineOptions.
Note: Currently cl::ParseEnvironmentOptions does not support quoting, so
an environment variable containing -option"foobar" will be parsed as three
words, -option, "foo, and bar", which is different from what you
would get from the shell with the same input.

The cl::SetVersionPrinter function is designed to be called directly from
main and beforecl::ParseCommandLineOptions. Its use is optional. It
simply arranges for a function to be called in response to the --version
option instead of having the CommandLine library print out the usual version
string for LLVM. This is useful for programs that are not part of LLVM but wish
to use the CommandLine facilities. Such programs should just define a small
function that takes no arguments and returns void and that prints out
whatever version information is appropriate for the program. Pass the address of
that function to cl::SetVersionPrinter to arrange for it to be called when
the --version option is given by the user.

The cl::opt class is the class used to represent scalar command line
options, and is the one used most of the time. It is a templated class which
can take up to three arguments (all except for the first have default values
though):

The first template argument specifies what underlying data type the command line
argument is, and is used to select a default parser implementation. The second
template argument is used to specify whether the option should contain the
storage for the option (the default) or whether external storage should be used
to contain the value parsed for the option (see Internal vs External Storage
for more information).

The third template argument specifies which parser to use. The default value
selects an instantiation of the parser class based on the underlying data
type of the option. In general, this default works well for most applications,
so this option is only used when using a custom parser.

This class works the exact same as the cl::opt class, except that the second
argument is the type of the external storage, not a boolean value. For this
class, the marker type ‘bool’ is used to indicate that internal storage
should be used.

The cl::alias class is a nontemplated class that is used to form aliases for
other arguments.

namespacecl{classalias;}

The cl::aliasopt attribute should be used to specify which option this is an
alias for. Alias arguments default to being cl::Hidden, and use the aliased
options parser to do the conversion from string to data.

The cl::extrahelp class is a nontemplated class that allows extra help text
to be printed out for the -help option.

namespacecl{structextrahelp;}

To use the extrahelp, simply construct one with a constchar* parameter to
the constructor. The text passed to the constructor will be printed at the
bottom of the help message, verbatim. Note that multiple cl::extrahelpcan be used, but this practice is discouraged. If your tool needs to print
additional help information, put all that help into a single cl::extrahelp
instance.

The cl::OptionCategory class is a simple class for declaring
option categories.

namespacecl{classOptionCategory;}

An option category must have a name and optionally a description which are
passed to the constructor as constchar*.

Note that declaring an option category and associating it with an option before
parsing options (e.g. statically) will change the output of -help from
uncategorized to categorized. If an option category is declared but not
associated with an option then it will be hidden from the output of -help
but will be shown in the output of -help-hidden.

Parsers control how the string value taken from the command line is translated
into a typed value, suitable for use in a C++ program. By default, the
CommandLine library uses an instance of parser<type> if the command line
option specifies that it uses values of type ‘type’. Because of this,
custom option processing is specified with specializations of the ‘parser’
class.

The CommandLine library provides the following builtin parser specializations,
which are sufficient for most applications. It can, however, also be extended to
work with new data types and new ways of interpreting the same data. See the
Writing a Custom Parser for more details on this type of library extension.

The generic parser<t> parser can be used to map strings values to any data
type, through the use of the cl::values property, which specifies the
mapping information. The most common use of this parser is for parsing enum
values, which allows you to use the CommandLine library for all of the error
checking to make sure that only valid enum values are specified (as opposed to
accepting arbitrary strings). Despite this, however, the generic parser class
can be used for any data type.

The parser<bool> specialization is used to convert boolean strings to a
boolean value. Currently accepted strings are “true”, “TRUE”,
“True”, “1”, “false”, “FALSE”, “False”, and “0”.

The parser<boolOrDefault> specialization is used for cases where the value
is boolean, but we also need to know whether the option was specified at all.
boolOrDefault is an enum with 3 values, BOU_UNSET, BOU_TRUE and BOU_FALSE.
This parser accepts the same strings as ``parser<bool>``.

The parser<string> specialization simply stores the parsed string into the
string value specified. No conversion or modification of the data is
performed.

The parser<int> specialization uses the C strtol function to parse the
string input. As such, it will accept a decimal number (with an optional ‘+’
or ‘-‘ prefix) which must start with a non-zero digit. It accepts octal
numbers, which are identified with a ‘0’ prefix digit, and hexadecimal
numbers with a prefix of ‘0x’ or ‘0X’.

The parser<double> and parser<float> specializations use the standard
C strtod function to convert floating point strings into floating point
values. As such, a broad range of string formats is supported, including
exponential notation (ex: 1.7e15) and properly supports locales.

Although the CommandLine library has a lot of functionality built into it
already (as discussed previously), one of its true strengths lie in its
extensibility. This section discusses how the CommandLine library works under
the covers and illustrates how to do some simple, common, extensions.

One of the simplest and most common extensions is the use of a custom parser.
As discussed previously, parsers are the portion of the CommandLine library
that turns string input from the user into a particular parsed data type,
validating the input in the process.

This approach has the advantage that users of your custom data type will
automatically use your custom parser whenever they define an option with a
value type of your data type. The disadvantage of this approach is that it
doesn’t work if your fundamental data type is something that is already
supported.

Write an independent class, using it explicitly from options that need it.

This approach works well in situations where you would line to parse an
option using special syntax for a not-very-special data-type. The drawback
of this approach is that users of your parser have to be aware that they are
using your parser instead of the builtin ones.

To guide the discussion, we will discuss a custom parser that accepts file
sizes, specified with an optional unit after the numeric size. For example, we
would like to parse “102kb”, “41M”, “1G” into the appropriate integer value. In
this case, the underlying data type we want to parse into is ‘unsigned’. We
choose approach #2 above because we don’t want to make this the default for all
unsigned options.

Our new class inherits from the cl::parser template class to fill in
the default, boiler plate code for us. We give it the data type that we parse
into, the last argument to the parse method, so that clients of our custom
parser know what object type to pass in to the parse method. (Here we declare
that we parse into ‘unsigned’ variables.)

For most purposes, the only method that must be implemented in a custom parser
is the parse method. The parse method is called whenever the option is
invoked, passing in the option itself, the option name, the string to parse, and
a reference to a return value. If the string to parse is not well-formed, the
parser should output an error message and return true. Otherwise it should
return false and set ‘Val’ to the parsed value. In our example, we
implement parse as:

boolFileSizeParser::parse(cl::Option&O,StringRefArgName,conststd::string&Arg,unsigned&Val){constchar*ArgStart=Arg.c_str();char*End;// Parse integer part, leaving 'End' pointing to the first non-integer charVal=(unsigned)strtol(ArgStart,&End,0);while(1){switch(*End++){case0:returnfalse;// No errorcase'i':// Ignore the 'i' in KiB if people use thatcase'b':case'B':// Ignore B suffixbreak;case'g':case'G':Val*=1024*1024*1024;break;case'm':case'M':Val*=1024*1024;break;case'k':case'K':Val*=1024;break;default:// Print an error message if unrecognized character!returnO.error("'"+Arg+"' value invalid for file size argument!");}}}

This function implements a very simple parser for the kinds of strings we are
interested in. Although it has some holes (it allows “123KKK” for example),
it is good enough for this example. Note that we use the option itself to print
out the error message (the error method always returns true) in order to get
a nice error message (shown below). Now that we have our parser class, we can
use it like this:

Several of the LLVM libraries define static cl::opt instances that will
automatically be included in any program that links with that library. This is
a feature. However, sometimes it is necessary to know the value of the command
line option outside of the library. In these cases the library does or should
provide an external storage location that is accessible to users of the
library. Examples of this include the llvm::DebugFlag exported by the
lib/Support/Debug.cpp file and the llvm::TimePassesIsEnabled flag
exported by the lib/IR/PassManager.cpp file.